Plasmon stabilized laser diodes

ABSTRACT

A device having a light cavity includes, at one end, a plasmonic reflector having a grating surface for coupling incoming light into traverse plasmon waves and for coupling the traverse plasmon wave into broaden light, the surface serving to redistribute light within the cavity, the reflector being well suited for use in laser diodes for redistributing filamental cavity laser light into spatially broaden cavity laser light for translating multimodal laser light into unimodal laser light for improved reliability and uniform laser beam creation.

PRIORITY CLAIM

This application is a continuation of U.S. patent application Ser. No.11/978,253 filed Oct. 29, 2007 and entitled PLASMON STABILIZED UNIMODALLASER DIODES.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the fields of multimodal laser diodes andpassive plasmonic devices. More particularly, the present inventionrelates to a laser diode facet that employs plasmonic coupling tolaterally redistribute the laser light.

2. Discussion of the Related Art

There has been recent progress in the guiding and focusing of lightbeyond the diffraction limit through the use of surface plasmons.Because of the potentials afforded by greatly increasing the opticalenergy density, plasmonic technology advances have created a new fieldknown as plasmonics. There is a good deal of significant prior art withmost of the work relating to applications such as near-field opticalmicroscopy, surface enhanced Raman spectroscopy, heat-assisted magneticrecording, and optical data storage. The prior art specifically relatingto patterning of metal films on the facets of laser diodes also exists.A. Partovi, et. al., teaches a “High-power laser light source fornear-field optics and its application to high-density optical datastorage,” in, Appl. Phys Lett., vol. 75, no. 11, pp. 1515-1517, 1999. F.Chen, et. al., teaches “Imaging of optical field confinement in ridgewaveguides fabricated on very-small-aperture laser,” in Appl. PhysLett., vol. 83, no. 16, pp. 3245-3247, 2003. E. Cubukcu, et. al., teacha “Plasmonic laser antenna,” in Appl. Phys Lett., vol. 89, pp.093120-1-3, 2006. These efforts were directed to increasing thenear-field intensity by focusing plasmonic waves at a single location onthe facet of single-lateral-mode laser diodes.

Nanorods have been grown on a surface to control an index of refraction.J. Q. Xi et. al., teaches “Optical thin-film materials with lowrefractive index for broadband elimination of Fresnel reflection.”, inNature Photonics, vol. 1, pp. 176-179, 2007. This technology is used tocreate a spatially graded index which would allow for wide-bandantireflection coatings. In addition to nanorods, plasmonic Bragggratings have been disclosed. A. Boltasseva et al., teaches “CompactBragg Gratings for Long-Range Surface Plasmon Polaritons,” in theJournal of Lightwave Technology, v. 24, no. 2, pp. 912-918, 2006. ABragg grating was used to reflect plasmonic waves of variousfrequencies. Thus, the plasmon is the input that hits the plasmonicgrating, and plasmons of certain frequencies are reflected while otherplasmons are transmitted. This Bragg grating functioned entirely in theplasmonic regime. Plasmonic gratings have also been made with very highefficiency of plasmon generation, while uncoupled light is specularlyreflected as in the conventional optical facet. Prior art Bragg gratingsare disadvantageously limited to the plasmonic regime without theability to expand, shape, or manipulate the modes of the plasmons.

The conventional high power laser diode does not fill its laser gaincavity during standard operation. Instead, the optical mode forms afilament due to the optical and gain dynamics of the device. Theposition of this lasing filament is not static but rather moves throughthe device, creating multimodal hot-spots and thermal lenses whichaccelerate failure of the device. These and other disadvantages aresolved or reduced using the invention.

SUMMARY OF THE INVENTION

An object of the invention is to provide a plasmonic reflector in alaser diode.

Another object of the invention is to distribute laser light in a laserdiode.

Yet another object of the invention is to provide a unimodal laser.

Still another object of the invention is to provide a high reliabilitydiode laser.

A further object of the invention is to distribute an electrical fieldin a laser diode to distribute laser light generation within the laserdiode.

The invention is directed to a large-area laser having a plasmonicreflector. The plasmonic reflector redistributes the lateral modeprofile within the laser diode to advantageously reduce the potentialfor filamented or confined lasing and the consequential hot spots withinthe laser diode. The plasmonic reflector serves to control the diffusionof the optical field, by redistribution, as being diametrically opposedto the focusing, for mode spoilage or mode expansion, or both. Thereflector is preferably a patterned metal film disposed in amulti-transverse-mode laser diode for the purposes of redistributing thereflected light for controlling both spatial, polarization and spectralparameters of the laser light. The reflector is used for generatingfree-space laser exit beams by expanding, shaping, or manipulating thelaser light within the laser diode.

The plasmonic optical reflector can employ a nano scale conductivesurface, allowing control of both spectral and spatial reflectivityproperties. The reflector can stabilize the cavity modes of multi-mode,high-power laser diodes. In one embodiment, the invention sits behindthe high-reflectivity facet of a multi-mode, high-power laser diode,which is patterned with metal films such that the near-field opticalradiation can be spatially and temporally stabilized. A reflective facetconsisting of a nano scale structured conductor selectivelyredistributes the spatial profile of the optical energy upon reflection.These patterned metal films may consist of nanorods, corrugatedsurfaces, round holes, square holes, or other patterns havingsubwavelength structures. The metal film serves as an optical antenna toredistribute and homogenize the optical radiation across the facet ofthe laser, or other resonant optical device, to better fill the lasercavity with laser light. By patterning the facet of a multimode,high-power laser diode with arrays of metallic nano-structures,deleterious effects, such as filamentation and catastrophic opticalmirror damage may be mitigated. More importantly, subwavelength controlof the optical field of the laser diode allows for manipulation of thestanding wave optical modes throughout the cavity for control of themodal distributions, output brightness, and light losses. Subwavelengthcontrol serves as a mode spoiler or mode shaper, as well as a controlover the polarization properties of the mode. The subwavelength controlminimizes random generation of localized hot spots at the facets andwithin the cavities for improving high-power laser diodes reliability.These and other advantages will become more apparent from the followingdetailed description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a plasmonic reflector.

FIG. 2 depicts a mode spoiling plasmonic laser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the invention is described with reference to thefigures using reference designations as shown in the figures. Referringto FIG. 1, a plasmonic reflector has a reflective grating surface thatmay be, for example, a square wave irregular surface of regularly spacedraised and lower portions. The irregular surface is a grating surface.The reflector receives a filamental optical wave and generates atraverse plasmon wave extending traversely through the reflectororthogonal to the incoming filamental optical wave. After traverselytraveling, the plasmon wave out-couples into a spatially broader opticalwave as a reflection. As such, the reflector serves to redistribute theoptical wave from a narrow multimodal filamental optical wave to aspatially broader optical wave. A broadened optical wave filling thelaser cavity is referred to as unimodal. The effect of the plasmonicreflector is to translate fine multimodal operation into broad unimodaloperation. A spatially localized optical filament, shown with dimensionsexaggerated for convenience, is incident upon the plasmonic reflector. Aportion of this filament is specularly reflected, as would be the casefor a standard metal reflecting surface. The remaining portion of theincident wave is coupled into surface plasmons due to the corrugatedstructure of the reflector. These plasmons propagate normal to theincident filament and are out-coupled into free space optical waves.This broadens the incident filament upon reflection.

Referring to FIGS. 1 and 2, and more particularly to FIG. 2, theplasmonic reflector is positioned on the high-reflectivity facet of aconventional semiconductor laser. The plasmonic reflector is preferablydisposed on a dielectric reflector of a convention laser. Because metalsgenerally cannot dissipate the energy absorbed in intense opticalfields, the plasmonic reflector is located behind the conventionalhigh-reflectivity dielectric coating. There are many possible methods offabricating this reflector on the back of a laser diode, includingphotolithographic patterning followed by an anisotropic etch and metaldeposition, or by a simple focused ion beam etch followed by metaldeposition. The plasmonic laser has internal laser light within asemiconductor laser cavity. The laser light has been spatially broadenedwithin the laser cavity. The reflector couples the normally incidentoptical fields into plasmons which will propagate traversely along thesurface of the metal plasmonic reflector at high reflective facet end.The plasmonic reflector serves to broaden the optical light throughplasmon and optical coupling. The coupling is firstly from thefilamental optical wave to the traverse plasmonic wave. The coupling issecondly from the transverse plasmonic wave to the broadened opticalwave. The optical and plasmonic coupling can be accomplished by manydifferent plasmonic reflector designs, such as the use of a plasmonicgrating reflector. A grating with 25% efficiency can be used to couplelight from the plasmon wave to a normal free space wave. By varying thegrating depth and period of the irregular surface, the exact couplingefficiency is determined by engineering parameters that can be set tooptimize the laser performance. The traverse plasmon wave will propagatenormally to the incident radiation and along the metal surface. Becausethe laws of physics are reversible and time-invariant, the same gratingwhich coupled light into the plasmonic field will couple that field backout into the laser cavity. In so doing, the plasmon wave laterallyredistributes the reflected beam and spread the optical energy tohomogenize the optical power distribution within the cavity. Surfaceplasmons are uniquely suited for redistribution because plasmonic wavevectors may be greater than those of the free-space optical field.Because of the greater wave vectors, the gratings of the plasmonicreflector can be constructed which only couples surface plasmons forreducing modes of the laser cavity.

The dielectric reflector defines a back facet of the laser diode that iscovered by a metal film that has been preferably patterned with an arrayof nano structured features. The fabrication of the irregular surfacecan be done in a variety of ways. For example, a thin metal film iselectron-beam evaporated or sputtered onto the facet of the device. Themetal film could be of any variety of metals that are known to supportand guide surface plasmons. The conductive metals such as copper, gold,aluminum and silver may be used. The metal film is then patterned on thenanometer scale. The metal film may be a directly deposited e-beam filmthat could form nano-structured droplets of metal forming the desiredirregular grating surface of the plasmonic reflector. The gratingsurface can also be made done etching patterns into a metal film or anunderlying dielectric film. Such etching can be done using focused ionbeams, nano-imprint lithography, or by any other lithographic orpatterning methods that can obtain nanometer resolution. Alternately,the metal film could be deposited by a focused ion beam into the desiredpattern. Direct write deposition would eliminate the need for a separateetching step.

The metal film can be patterned in various geometries. In the preferredembodiment, a one or two dimensional grating can be etched into theunderlying dielectric film of the facet followed by evaporation of ametal film. The grating would serve to couple light from the lasercavity to surface plasmon modes of the metal film. Because thewave-vector of surface plasmons is greater than that of free space lightof the same frequency, the grating could be constructed such that thegrating would only couple from normally incident radiation intoplasmons, and then from the plasmons back into the cavity optical modes.The grating structure would then mitigate losses because the grating isthen only capable of scattering between the plasmonic modes and cavityoptical modes. Therefore, scattering to modes outside the cavity iseliminated. The grating could be chirped or optimally patterned so thatpoints of high optical intensity at the periphery of the facet aredirected towards the center of the facet. With proper design andmodeling, an optimal end-facet geometric structure can be realized thatcontrols and more evenly distributes the optical intensity within thecavity, while spoiling any single preferential spatial mode, within amultimode laser cavity. Alternatively, the underlying facet gratingsurface can be roughened with gallium beams using focused ion beamdeposition. A thin metal film is evaporated onto that roughened surface.The rough metal film couples the light in the laser cavity to surfaceplasmon modes in the metal through a matching of the wave vector. In thereverse process, the surface plasmon wave can reradiate light back intothe laser cavity for redistributing the optical light.

The invention is directed to an optical device having a cavity throughwhich optical light is passed but having a modified optical mode using aplasmonic reflector for spatially redistributing intensity profiles wellsuited for high-power laser diodes end facets. In a broad aspect, thedevice is used for redistributing optical light using an optical cavityhaving two ends. The reflector at one end is for receiving the light asincoming light and for emitting the light as exiting light. Thereflector has a surface for coupling the received light into a traverseplasmon wave. The surface is further for coupling the plasmon wave intothe exiting light communicated through the cavity to the second end.

Preferably, the device is a laser for generating laser light with thereflector receiving incoming light as filamental laser light andproviding exiting light that is broadened laser light. The optionaldielectric film is disposed between the reflector and the cavity forelectrically isolating the reflector from the cavity. In the preferredform, the reflector surface is a grating. The grating is made from aconducting metal. Plasmonic reflectors can be used for improving beamquality, reliability, and robustness of a device having an opticalcavity, such as broad-area laser diodes. The plasmonic reflectormodifies the lateral optical mode profile of an optical device, such asa laser or resonator. The mode profile can be controlled through theincorporation of a plasmonic reflector having nanostructured metalfilms. The nanostructured film functions by coupling the incomingradiation into modes of the metal which propagate energy perpendicularto the axis of the device cavity. These modes can be either travelingwave modes or coupled resonators, both of which serve to redistributeoptical energy. The same mechanism which in-couples the light into themodes of the metal also serves to out-couple it back into the cavity,only now it has been translated by some engineered length. This wouldhomogenize the energy distribution and minimize the build-up ofhot-spots and could spoil higher order modes or undesired polarizations.These nanostructures could be implemented as arrays of nano-particles,square or round holes in a continuous metal film, a roughened surfacecoated with a thin metal, a one-dimensional or two-dimensional gratingstructure, quantum dots, and other structures having dimensions lessthat the wavelength of laser light. Various plasmonic reflector gratingscan be used to redistribute optical light within a device cavity. Thoseskilled in the art can make enhancements, improvements, andmodifications to the invention, and these enhancements, improvements,and modifications may nonetheless fall within the spirit and scope ofthe following claims.

1. A device for spreading optical light, the device comprising: amulti-transverse mode laser including a lasing cavity; a first end ofthe lasing cavity next to a high reflectivity dielectric reflector; asecond end of the lasing cavity for emitting light; the first and secondends of the cavity lying in substantially parallel planes; thehigh-reflectivity dielectric reflector interposed between the lasingcavity first end and a plasmonic reflector; the plasmonic reflector forreceiving light as incoming light and for emitting the light as exitinglight; a reflective grating of the plasmonic reflector next to the highreflectivity dielectric reflector; the reflective grating for couplingthe incoming light into a transverse plasmon wave; and, the reflectivegrating further for coupling the plasmon wave into the exiting lightcommunicated through the cavity to the second end.
 2. The device ofclaim 1 wherein the device is a laser, the light is laser light, theincoming light is filamental light, and the exiting light is spatiallybroadened light.
 3. The device of claim 1, wherein, the reflectivegrating is made by depositing a film over an irregular surface.
 4. Thedevice of claim 1, wherein, the reflector is made of a conducting metalselected from the group consisting of gold, copper, aluminum, andsilver.
 5. The device of claim 1, wherein, the plasmonic reflector ismade of metal and the reflective grating comprises structures smallerthan the wavelength of the laser light.
 6. The device of claim 1,wherein, the surface is made of features selected from the groupconsisting of lines, dots, and bumps.
 7. The device of claim 1, wherein,the device is a laser, the laser is a plasmonic laser, the light islaser light, the incoming light is spatially filamental laser light, andthe exiting light is spatially broadened laser light.
 8. The device ofclaim 7, wherein, the plasmonic reflector is made of metal.
 9. Thedevice of claim 7, wherein, the plasmonic reflector is made of aconducting metal.
 10. A device for reducing laser light filamentation,the device comprising: a multi-transverse mode laser including a lasingcavity; a first end of the lasing cavity next to a high reflectivitydielectric reflector; a second end of the lasing cavity for emittinglight; the first and second ends of the cavity lying in substantiallyparallel planes; the high-reflectivity dielectric reflector interposedbetween the lasing cavity first end and a plasmonic reflector; theplasmonic reflector for receiving light as incoming light and foremitting the light as exiting light; a reflective grating of theplasmonic reflector next to the high reflectivity dielectric reflector;the reflective grating for coupling the incoming light into a transverseplasmon wave; the transverse plasmon wave operative to reduce laserlight filamentation by spatially redistributing light; and, thereflective grating further for coupling the plasmon wave into theexiting light communicated through the cavity to the second end.
 11. Adevice for reducing laser hot spots, the device comprising: amulti-transverse mode laser including a lasing cavity; a first end ofthe lasing cavity next to a high reflectivity dielectric reflector; asecond end of the lasing cavity for emitting light; the first and secondends of the cavity lying in substantially parallel planes; thehigh-reflectivity dielectric reflector interposed between the lasingcavity first end and a plasmonic reflector; the plasmonic reflector forreceiving light as incoming light and for emitting the light as exitinglight; a reflective grating of the plasmonic reflector next to the highreflectivity dielectric reflector; the reflective grating for couplingthe incoming light into a transverse plasmon wave; the transverseplasmon wave operative to reduce laser hot spots; and, the reflectivegrating further for coupling the plasmon wave into the exiting lightcommunicated through the cavity to the second end.